Organic materials have recently shown promise as the active layer in organic based thin film transistors and organic field effect transistors (OFETs) [see H. E. Katz, Z. Bao and S. L. Gilat, Acc. Chem. Res., 2001, 34, 5, 359]. Such devices have potential applications in smart cards, security tags and the switching element in flat panel displays. Organic materials are envisaged to have substantial cost advantages over their silicon analogues if they can be deposited from solution, as this enables a fast, large-area fabrication route.
The performance of the device is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2V−1 s−1). In addition, it is important that the semiconducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance.
In prior art regioregular head-to-tail (HT) poly-(3-alkylthiophene), in particular poly-(3-hexylthiophene), has been suggested for use as semiconducting material, as it shows charge carrier mobility between 1×10−5 and 0.1 cm2 V−1 s−1. Also, poly-(3-alkylthiophene) shows good solubility in organic solvents and is solution processable to fabricate large area films.
A high regioregularity of the poly-(3-alkylthiophene) is important for its electronic and photonic properties, as it leads to improved packing and optimised microstructure, leading to improved charge carrier mobility [see U.S. Pat. No. 6,166,172, H. Sirringhaus et al., Science, 1998, 280, 1741-1744; H. Sirringhaus et al., Nature, 1999, 401, 685-688; and H. Sirringhaus, et al., Synthetic Metals, 2000, 111-112, 129-132]. The regioregularity is strongly influenced by the method of preparing the polymer.
Several methods to produce highly regioregular HT-poly-(3-alkylthiophene) have been reported in prior art, for example in the review of R. D. McCullough, Adv. Mater.,1998, 10(2), 93-116 and the references cited therein.
Known methods to prepare HT-poly(3-alkylthiophene) with a regioregularity ≧90% starting from 2,5, dibromo-3-alkylthiophene include for example the “Rieke method”, by reacting the educt (wherein R is alkyl) with highly reactive zinc in THF as illustrated below.

Also known is the method described in McCullough et al., Adv. Mater., 1999, 11(3), 250-253 and in EP 1 028 136 and U.S. Pat. No. 6,166,172, the entire disclosure of these documents being incorporated into this application by reference. According to this route the educt is reacted with methylmagnesium bromide in THF as shown below.

It was also reported to prepare regioregular poly(3-alkylthiophene) by the “Stille-method” (see Stille, Iraqi, Barker et al., J. Mater. Chem., 1998, 8, 25) as illustrated below
or by the “Suzuki-method” (see Suzuki, Guillerez, Bidan et al., Synth. Met., 1998, 93, 123) as shown below.

However, the methods described in prior art have several drawbacks. Thus, for example the Rieke method requires the costly and difficult preparation of the highly active “Rieke zinc”. The Stille and Suzuki methods require an extra processing step which reduces the process efficiency. The McCullough method requires the costly Grignard reagent methylmagnesium bromide. Additionally, it produces methyl bromide as a by-product in stoichiometrical quantities which especially in large scale production causes environmental problems. As methyl bromide cannot be eliminated from the exhaust gases with a gas washer, costly techniques and means for exhaust air combustion are needed.
Prior art also reports the preparation of unsubstituted polythiophene by Nickel-catalysed coupling of a Grignard reagent that is formed from the reaction of a 2,5-dihalogenated thiophene with magnesium.
This route was first reported in 1984 (J. P. Montheard; T. Pascal, Synth. Met, 1984, 9, 389 and M. Kobayashi; J. Chen; T.-C. Chung; F. Moraes; A. J. Heeger; F. Wudl, Synth. Met, 1984, 9, 77). However, this method yielded only low molecular weights. Also these polymers only have low solubility, compared to 3-alkyl substituted polythiophenes.
There are also reports on the preparation of poly (3-alkyl thiophene) using the Nickel-catalysed coupling of a Grignard reagent formed from 2,5-dihalogenated 3-alkylthiophene with magnesium as illustrated below.

T. Yamamoto; K.-I. Sanechika; A. Yamamoto, Bull. Chem. Soc. Jpn., 1983, 56, 1497 and U.S. Pat. No. 4,521,589 describe the Grignard coupling of 2,5-dihalo-3-alkylthiophene in THF, wherein the alkyl group is lower alkyl with 1 to 4 C-atoms, like methyl. However, the resulting polymers are reported to have low molecular weight (1,370 or 2,300) and low regioregularity (as can be seen from the 1H—NMR spectra). U.S. Pat. No. 4,521,589 also mentions a higher weight fraction with a degree of polymerization of 96 as concluded from IR spectra, but no molecular weight measurement data are given. A series of publications by Elsenbaumer et al. also describes the synthesis of poly (3-alkyl thiophenes) by Grignard coupling (R. L. Elsenbaumer; K. Y. Jen; R. Oboodi, Synth. Met, 1986, 15, 169; furthermore K. Y. Jen; R. Oboodi; R. L. Elsenbaumer, Polym. Mater. Sci. Eng., 1985, 53, 79 and K.-Y. Jen; G. G. Miller; R. L. Elsenbaumer, J Chem Soc, Chem Commun, 1986, 1346). However, these polymers mostly have molecular weights (Mn) in the range of 3,000 to 8,000, with the exception of a homopolymer obtained from 2,5-diiodo-3,4-dimethylthiophene and a copolymer obtained from 2,5-diiodo-3-methyl-and 2,5-diiodo-3-n-butylthiophene, which are reported to have a molecular weight of 26,000 and 35,000 respectively. U.S. Pat. No. 4,711,742 (Elsenbaumer et al.) reports the synthesis of poly(3-butylthiophene) by Grignard coupling of the diiodo monomer in 2-methyl-tetrahydrofuran, giving a molecular weight of 41,400 corresponding to a degree of polymerization of 300. However, the poly (3-alkyl thiophenes) described in the above documents are regiorandom with relatively low amounts of the desired head-to-tail head-to-tail (HT-HT) triads (see e.g. K.-Y. Jen; G. G. Miller; R. L. Elsenbaumer, J Chem Soc, Chem Commun, 1986,1346).
In a study of poly (3-decyl thiophene) made by three different routes (see P. C. Stein; C. Botta; A. Bolognesi; M. Catellani, Synth. Met, 1995, 69, 305) a sample prepared using the Grignard polymerisation of the diiodo monomer was found to have a regioregularity of 70% by 1H—NMR. A study of the synthesis of poly (3-hexyl thiophene) using the Grignard reaction of 2,5-diiodo-3-hexylthiophene with magnesium in ether (see H. Mao; S. Holdcroft, Macromolecules, 1992, 25, 554) resulted in a molecular weight (Mn) of 5,200. No direct regioregularity data are given but the 1H—NMR shows peaks from all four possible triads, suggesting relatively low regioregularity. Another study of the same authors (see H. Mao; S. Holdcroft, Macromolecules, 1993, 26, 1163) reported only poly (3-hexyl thiophenes) with low regioregularities (up to 58% HT-HT triads or up to 80% HT dyads).
In conclusion, although the preparation of poly (3-alkyl thiophenes) by Grignard reaction with magnesium has been described in the literature, the polymers synthesised are reported to have low molecular weight and/or low or random regioregularity.
Therefore, there is still a need for an improved method of preparing polymers, in particular poly-(3-substituted) thiophenes with high regioregularity, high molecular weight, high purity and high yields in an economical, effective and environmentally beneficial way, which is especially suitable for industrial large scale production.
It was an aim of the present invention to provide an improved process for preparing polymers with these advantages, but not having the drawbacks of prior art methods mentioned above.
Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
The inventors of the present invention have found that these aims can be achieved, and the above problems be solved, by providing a process of preparing polymers, in particular poly-(3-substituted) thiophenes according to the present invention as described below. According to this process, a 3-substituted thiophene monomer with at least two groups, wherein these groups are leaving groups that are capable of reacting with magnesium, is reacted with magnesium in the presence of a catalytic amount of an organohalide or organomagnesium halide in a suitable solvent to form an intermediate, which is then polymerized in the presence of a suitable catalyst. It was surprisingly found that, by using this method it is possible to obtain polymers, in particular poly-(3-substituted) thiophenes, with high regioregularity, high molecular weight and high purity in good yields and avoiding large amounts of hazardous by-products that need to be eliminated.
The polymers prepared by the process according to the present invention are especially useful as charge transport materials for semiconductor or light-emitting materials, components or devices.